Embodiments of the inventive subject matter generally relate to the field of communication systems, and, more particularly, to adjusting gain of a transmission signal in view of digital to analog converter (DAC) illumination.
A transmitter of a communication device may include a variety of components, referred to as a transmit chain. Several components of a transmit chain can amplify a transmission signal to produce a desired output power. For example, the transmitter may use analog amplifiers to boost the transmission signal. An amplified transmission signal may result in a higher quality signal (e.g., a signal with high signal-to-noise ratio, SNR). Higher SNR may be especially useful for data frames that are composed of high levels of quadrature amplitude modulation (QAM). However, amplifying the analog signal may enhance quantization error or other imperfections of the analog signal.
In a transmitter of a communication device, source signals in the form of a digital baseband signal may be converted to an analog signal at a digital-to-analog converter (DAC). The power of the digital baseband signal may impact the DAC illumination (sometimes referred to as an “operating point” of the DAC). DAC illumination refers to an amount of amplitude scaling (or amplification of the signal power level) of a digital input signal going into a DAC. The digital input signal is “illuminated” to increase the amplitude of the signal power in the digital domain, such that the DAC operates on the digital input signal at a particular average amplitude level. In other words, a digital input signal will be digitally amplified (“illuminated”) so that the power range of the digital input signal improves the digital-to-analog conversion of the digital input signal to an analog output signal. The DAC illumination (and the DAC operating level) may be referenced by a root mean square amplitude level of the digital input signal. The root mean square amplitude can be estimated as the standard deviation of a signal packet which may have a zero mean value.
A DAC typically has an optimal DAC illumination based on the design of the transmitter circuitry. For example, a fully utilized DAC should receive a source signal such that each of the DAC input bits contributes to the output at the DAC. However, operating a DAC at a maximum of the digital range of the DAC (e.g., maximum DAC illumination) may introduce signal clipping in the event of a burst of digital data. Signal clipping may cause the signal SNR to drop rapidly if signal amplitude is increased, may cause spectral distortion of the transmitter output, or may cause a reduction in notching capabilities of the transmitter. Conversely, operating a DAC with signals at a low level of the digital range of the DAC (e.g., low DAC illumination) may be an inefficient use of the DAC and signal resources. Furthermore, DAC illumination may be too low, which may not provide a sufficient signal-to-noise ratio due to quantization noise.